Add the following to your Custom HTML:

<script type="text/javascript">fbComments("light", "560", "15");</script>​
There are three customizable options: ("color", "width", "comment count")

Color can be set to light or dark, depending on the background of your site. Comment count defines how many comments are displayed before pagination. If you wish to adjust the margins, add this class to your Custom CSS:

.fb_comments { margin: 30px 0 30px 0 !important; }​